[skydawg]

What's the latest? Hope you figured it out.

One more thing to look at if still not working.... many of the alternators of this vintage required a field or sense wire that read ignition voltage rather than from the battery peg. If there was no voltage on this wire, there was no output. So, your 12.5 v would drop after starting the engine (due to starter load), and volts would not come up even though alternator was spinning (bat was depleted from start and now powering car load). This was a typical problem when owners would think the alternator was bad and have it rebuilt, and it would check good on a test stand, but still bad on actual car. If there is no voltage on field or sense wire/terminal, jump +12v to it and check for output to be sure that's the issue. I don't have a wire diagram for your car but someone here does that can tell you wire colors.

If there is higher voltage coming from alternator but not getting to rest of car circuits, Would suggest borrowing a DC clamp amp multimeter (make sure it has DC load option) and check amp load on the alternator. I would expect to see about 24-35 amps after start then settle about 12 amps in a few minutes with engine running. If way higher, this could explain the low volts and some voltage regulators have protection circuits that lower output with high draw or high temperatures. Good luck.

 

[skydawg]

Hummm... this is a simple circuit. Just so I am clear, the alternator produces over 13-14V on test stand but not in car

the voltage regulator is not getting the right power or ground. The lamp circuit actually plays a vital role in the regulator..... when IGN power is ON the + side is energized and the other lamp wire (-) from the alternator; but this - become + as the alternator produces energy so the bulb has + on both sides and the light extinguishes (the + then powers part of the regulator circuit). If the lamp is not functioning or burned out or bulb has wrong resistance rating, it could effect regulator circuit not to work).

Please try following and let me know how it went.
1. check cable between bat and alternator for good connection and corission. Be shure bat is fully charged.
2. I'm not familiar with your exact alternator but there should either be a small connector or individual wires that attach to the alternator..... I don't trust Chilton diagrams for wire diagrams, but ask on forum if someone has Isuzu manual diagram...
.... 2A. with key in ON position, check volts on IGN+ at the alternator; you should have bat voltage within .3V (if you do, jump to step 2) If not, jump alternator to bat+ terminal and start engine and start engine. . Are you now getting above 13V at bat with engine running? If so, trace the IGN wire to find fault; it should have about bat + volts with key in start or run. .

3. If above circuit diagram is correct, beside IGN+ there should be a wire for dummy light. This wire coming from the alternator (actually its from regulator that's mounted inside alternator) should be grounded with engine off..... test for near zero resistance between this wire and a known ground such as the engine block of bat - terminal. If this wire is not grounded the alternator or ground wire is not connected to engine block (some alternators are isolated case meaning the case is not connected to regulator - circuit and there is a separate - grounding wire. Be sure any grounding wire is tightly connected to a good ground.

3b. Above diagram shows a white/yellow for another IGN power input (IGN= ignition meaning wire is powered with key in run or start position)....check for battery + volts at alternator for this wire. If not, check fuse and wire for volts with key ON


4. the lamp wire needs a certain amount of resistance to complete its circuit, or some a diode. Check if the alternator lamp is good, if not replace it and check if this solve issue. the bulb should have bat + volts to it with key ON on one wire, and other wire (coming from the alternator) should be -, this causes bulb to light up. Once the alternator is producing energy, the - wire becomes + or switches to open causing bulb to turn off, but most alternators require some resistance in this circuit, which the bulb performs. A burned out or wrong size bulb or LED won't work.

5. if everything above checks out, and the diagram above is correct, check the relays function (you can easily do this by putting finger on relay and feel and hear a click when the key is turned on). or swap it for other relay you know works. If the good relay doesn't work in the alternator plug when turning key on, check the ground (BLK) and IGN + (white/violet) for the coil at the spade terminal the relay plugs into (likely black wire or relay connections are noted on relay)... ground wire should have very little resistance with known ground such as engine block.